The atmospheric concentration of the greenhouse gas carbon dioxide has reached a new high, say US researchers.

The figures - 378 parts per million (ppm) - were gathered by a Hawaiian lab regarded by experts as one of the most reliable in climate research.

The rise in the past year is smaller than it was in the previous two years.

But the trend remains upwards, as it has for every year since measurements began on top of the Mauna Loa volcano nearly half a century ago.

The research was carried out by the US government's Climate Monitoring Diagnostics Laboratory, part of the National Oceanic and Atmospheric Administration (Noaa).

A good mix

The laboratory's director, Dr Pieter Tans, told the BBC: "The most striking thing about the data is that we've seen an increase in carbon dioxide levels every single year since 1958."

At an altitude of 3,500m (11,500ft), the research station must rank as one of the world's most spectacular and most remote scientific outposts.

Reaching it involves leaving the tropical heat and humidity of the Hawaiian coast and climbing up a narrow road that twists through barren fields of solidified lava.

Dr Pieter Tans: Concentration keeps rising

The thin Pacific air is ideal for this research since it is "well-mixed", meaning that there is no obvious nearby source of pollution, such as a heavy industry, or a natural "sink", such as forest which would absorb CO2.

For that reason the data from Mauna Loa has come to be seen as the benchmark by which atmospheric data is judged.

According to Dr Tans, one significant finding is that the annual rate at which the CO2 is rising has itself increased.

The growth rate over the past decade was about twice as fast as that found in the 1960s.

He says that variations in the growth rate year by year can be explained by natural factors; for example, changes in the rate at which plants and the oceans soak up carbon dioxide.

But he and his colleagues conclude that the steady rise overall can be attributed to man-made emissions of carbon.